The present invention relates to an air-conditioning system that can be operated with CO2 for a vehicle, having a high-pressure section which is connected to the output of a compressor and contains a gas cooler, having a low-pressure section which is connected to the suction side of the compressor and contains an evaporator, and having an expansion member connecting the high-pressure section and the low-pressure section.
In order to achieve an optimum cooling performance and an optimum performance figure in air-conditioning systems operated with CO2 (refrigerant R744), the known air-conditioning systems are provided with complicated controllers that control not only the compressor, but also the high pressure by means of an adjustable, expensive expansion valve, for which a high-pressure sensor is also needed. The high-pressure controller is considered to be necessary by those skilled in the art since the required cooling performance, which depends on the temperature of the external environment and on the desired cooling, can be implemented with an optimum performance figure at optimum high pressures associated with these performance figures. By means of the controlled expansion valve, it is desired to operate the air-conditioning system under any operating conditions at the high pressure at which the optimum of the cooling performance and the optimum of the performance figure (COP) is located. This achieves a situation where the air-conditioning systems operated with CO2 are of equal value to or, to some extent are even superior to air-conditioning systems that are operated with other refrigerants, for example the refrigerant R134a. Although the air-conditioning systems operated with CO2 are recognized to be more environmentally friendly than the air-conditioning systems operated with other refrigerants, considerably increased production costs oppose their practical implementation.
The present invention is based on the object of providing an air-conditioning system of the type described above whose production costs are reduced.
This object is achieved in that the expansion member provided is a fixed restrictor whose throttle opening is matched in terms of length and diameter to the system in such a way that under all operating conditions the pressure in the high-pressure section is limited to values of less than 14 MPa.
Surprisingly, it has been found that an air-conditioning system having a fixed restrictor designed in this way can be operated in the vicinity of the optima of cooling performance and performance figure, so that it is possible to dispense with an expensive expansion valve and its control. As a result, a considerable reduction in price can be implemented without noticeable penalties in relation to cooling performance and performance figure having to be tolerated.
In a further refinement of the invention, a pressure limiting valve connected in parallel with the fixed restrictor is provided. The pressure limiting valve ensures that the maximum pressure occurring is limited to a value which lies somewhat above the pressure value set by the fixed restrictor, for example at 16 MPa. In an advantageous embodiment, provision is made for the pressure limiting valve and the fixed restrictor to be designed as one structural unit. This permits a relatively simple and correspondingly cost-effective expansion member to be provided.
In a further refinement of the invention, provision is made for the fixed restrictor to contain a switchable throttling element having at least two different throttle openings. The switchable throttling element may be adjusted, for example, as a function of the pressure and/or the temperature, so that in each case use is made of a suitable throttle opening having a predefined throttle cross section. It is therefore possible, by means of a simple controller to improve the air-conditioning system a little further in relation to the optima of cooling performance and performance figure. The same effect may be achieved if provision is made for two or more fixed restrictors connected in parallel which can be brought into use by means of an associated valve.